1. Field of the Invention
The present invention relates to a novel titanyloxyphthalocyanine crystal, a method for preparing such crystal, and an electrophotographic photoreceptor that comprises the titanyloxyphthalocyanine crystal as a charge-generation material.
2. Description of the Prior Art
After inventing a principal theory of the process of electrophotography by C. F. Carlson in 1937, the electrophotographic technologies have been dramatically developed and applied in the devices for processing information, such as digital photocopying machines, laser printers, light-emitting diode printers, and facsimile machines.
The electrophotography is an image-forming method that depends on a skillful combination of photoconductive characteristics and electrostatic phenomena of the electrophotographic photoreceptor(C. F. Carlson, U.S. Pat. No. 2,297,691, 1942) That is, the electrophotography is characterized by the process comprising the steps of:
charging a surface of the electrophotographic photoreceptor by subjecting to colona discharge in darkness; PA1 producing an electrostatic latent image including characters, pictures, and the like in an original by exposing the charged surface of the photoreceptor to light; PA1 visualizing (i.e., developing) the electrostatic latent image by using toner (i e., colored and charged particles) that adheres to the charged surface of the photoreceptor for transforming the latent image into a real image; and PA1 transferring the developed toner image to a surface of substrate (e g., a sheet of paper) and fixing the image thereon. PA1 (1) the phthalocyanine pigment is chemically and physically stable and in general used as a color pigment of color ink, coating compound, or the like; PA1 (2) the phthalocyacine pigment is easily prepared; PA1 (3) the phthalocyanine pigment has the ability of absorbing light at the wavelengths of ending up to the long wavelengths of near infrared ray; and PA1 (4) the light absorption wavelengths and photo-conductivities of the phthalocyanine pigment are depended on the types of its central atom, crystal form, and preparation method, so that the big variations in these characteristics of the phthalocyanine can be occurred. PA1 (1) Titanyloxyphthalocyanine crystals classified in a first group are thermodynamically stable and can be obtained immediately after their syntheses. The X-ray diffraction spectrum of each crystal is characterized by having the maximum intensity of diffraction at the Bragg angle (2.theta.) of 26.3.degree.(.+-.0.2.degree.). PA1 (2) Titanyloxyphthalocyanine crystals classified in a second group are such as crystals in the typeof II as disclosed in Japanese Patent Application Laying-open No. 62-67094. The X-ray diffraction spectrum of each crystal is characterized by having the maximum intensity of diffraction at a Bragg angle (2.theta.) of 27.3.degree.(.+-.0.2.degree.). PA1 (3) Titanyloxyphthalocyanine crystals classified in a third group are such as crystals in the type of a as disclosed in Japanese Patent Application Laying-open No. 61-217050. The X-ray diffraction spectrum of each crystal is characterized by having the maximum intensity of diffraction at a Bragg angle (2.theta.) of 7.5.degree.(.+-.0.2.degree.). PA1 (4) Titanyloxyphthalocyanine crystals classified in a fourth group are such as crystals disclosed in Japanese Patent Application Laying-open No. 5-320167. The X-ray diffraction spectrum of each crystal is characterized by having the peak intensities of diffraction at Bragg angles (2.theta.) of 9.degree., 5.degree., 14.1.degree., 17.8.degree., 27.1.degree., and 29.0.degree. and the maximum intensity of diffraction at a Bragg angle (2.theta.) of 9.5.degree.(.+-.0.2.degree.). PA1 dispersing a predetermined amount of amorphous titanyloxyphthalocyanine in which an aqueous solution in which ionic substances are dissolved, under mechanical force at a temperature of under 50.degree. C., to obtain a mixture comprising fine dispersions; PA1 adding a non-aqueous organic solvent to the mixture to shift a dispersion of titanyloxyphthalocyanine from an aqueous phase to a non-aqueous organic solvent phase; PA1 removing the non-aqueous organic solvent phase to obtain a pellet of titanyloxyphthalocyanine crystal that has a maximum diffraction intensity observed at Bragg angle (2.theta.) of 9.60.degree..+-.0.2.degree. and clear peaks of diffraction intensity observed at 7.22.degree..+-.0.2.degree., 9.60.degree..+-.0.2.degree., 11.60.degree..+-.0.2.degree., 13.40.degree..+-.0.2.degree., 14.88.degree..+-.0.2.degree., 18.34.degree..+-.0.2.degree., 23.62.degree..+-.0.2.degree., 24.14.degree..+-.0.2.degree., and 27.32.degree..+-.0.2.degree., respectively, in a X-ray diffraction spectrum obtained by performing a X-ray diffraction method using CuK.alpha. as a source of radiation. PA1 a=16.3058 .ANG., b=23.078 .ANG., c=8.7155 .ANG., PA1 .alpha.=101.352.degree., .beta.23.078.degree., and .gamma.=117.530.degree., PA1 the charge generation material is a titanyloxyphthalocyanine crystal having a lattice constant of: PA1 a=16.3058 .ANG., b=23.078 .ANG., c=8.7155 .ANG., PA1 .alpha.=101.352.degree., .beta.-23.078.degree., and .gamma.=117.530.degree., PA1 dispersing a predetermined amount of amorphous titanyloxyphthalocyanine in which an aqueous solution in which ionic substances are dissolved, under mechanical force at a temperature of under 50.degree. C., to obtain a mixture comprising fine dispersions; PA1 adding a non-aqueous organic solvent to the mixture to shift a dispersion of titanyloxyphthalocyanine from an aqueous phase to a non-aqueous organic solvent phase; PA1 removing the non-aqueous organic solvent phase to obtain a pellet of titanyloxyhthalocyanine crystal that has a lattice constant of: PA1 a=16.3058 .ANG., b=23.078 .ANG., c=8.7155 .ANG., PA1 .alpha.=101.352.degree., .beta.-23.078.degree., and .gamma.=117.530.degree.,
Then the photoreceptor is discharged and cleaned of any excess toner using coronas, lamps or the like for recycling the photoreceptor.
In accordance with the above electrophotography, it is possible to digitize an original image of document by scanning and exposing a surface of the photoreceptor to light emitted from a light source such as a semiconductor laser or a light-emitting diode (LED). Therefore, an input image information can be recorded by converting an output of the data-processing device into a plurality of dots of light, and scanning and exposing the surface of the photoreceptor by using these dots. As the information-processing has been advanced and sped up in recent years, the data-processing devices (such as laser printing machines and digital copying machines which perform the high-qualified image formation at a predetermined high printing speed) have been sprung into wide use. Especially in the case of using the semiconductor laser as a source of exposure light in the above information-processing device, it is not only possible to provide a comparatively small and cheap device but also possible to provide the device having a high reliability and a high image quality. Thus the photoreceptor has been investigated and developed so as to have an excellent sensitivity against the light emitted from the semiconductor laser to be used as the above light source.
Currently, by the way, a wavelength of the light generated by the semiconductor laser in practical use has been limited to in the range of comparatively long wavelengths of near infrared ray (i.e., about 700-800 nm) Consequently, it has been required that the photoreceptor to be applied in the device that uses a semiconductor laser as an exposure light source should be of a high sensitivity against the above wavelengths. In addition, it has been also required that such photoreceptor should have the properties of being stable in repeat use and in various environmental conditions. That is, the needs for preventing the changes in electrical characteristics of the photoreceptor, the qualities of output images, the generation of memory, and the like are constantly increased in repeat use.
For the above requests, therefore, charge-generation materials have been studied and developed. In general, the following materials are known as the charge-generation materials: polyazo pigment, phthalocyanine pigment, azulenium base pigment, pyrylium pigment, and naphthoguinone pigment. However, the naphthoquinone pigment is of no practical use because of its insufficient photosensitivity. In case of the azulenium base pigment and the pyrylium pigment, their chemical constructions are unstable under a strong light such as a laser beam. In case of the phthalocyanine pigment, on the other hand, it has the following advantages compared with the others. That is,
From the above point of view, therefore, the phthalocyanine pigment can be provided as a preferable charge generation material of an electrophotographic photoreceptor.
The following pigment compounds have been studied for finding an appropriate type of the phthalocyanine pigment to be used as the charge generation material. For example, .epsilon.-type copper phthalocyanines, X-type nonmetal phthalocyanines, t-type nonmetal phthalocyanines, chloroaluminium phthalocyanine, chloroindium phthalocyanine, chlorogallium phthalocyanine, chloro-germanium phthalocyanine, vanadyloxy phthalocyanine, and titanyloxy phthalocyanine have been studied. Among these pigments, the titanyloxyphthalocyanine has attracted a great deal of public attention because of the following reasons. That is, the nonmetal phthalocyanines and the divalent metallic phthalcyanines do not have sufficient photo-sensitivities against the laser beam at the wavelengths of near infrared region. In addition, the trivalent and quadrant metallic phthalocyanines have disadvantages that their electric characteristics are changed in accordance with the progress of hydrolysis in which their chemical compounds are gradually decomposed by the reaction with water in the atmosphere. However, the quadrant metallic phthalocyanine having the metal connected with oxygen atom shows stable electric characteristics. Especially in recent years, therefore, titanyloxyphthalocyanine has been energetically studied for providing it as a charge-generation material and disclosed in the documents, for example Japanese Patent Application Publication No. 49-4338 that discloses titanium phthalocyanine as an example of metal phthalocyanine to be used as a photoconductive material of a resin-dispersed photoreceptor in the type of having a photosensitive layer formed as a single layer (hereinafter, it will be referred as a mono-type photoreceptor).
By the way, a laminate-type photoreceptor having a photosensitive layer formed as a laminate of functionally distinguishable layers (a charge generation layer and a charge transport layer) shows excellent electrophotographic properties compared with that of the mono-type photoreceptor. Accordingly, the above described titanyloxyphthalocyanine have been investigated as a material to be applied in a charge-generation layer of the laminate-type photoreceptor as described below.
In the documents of Japanese Patent Application Laying-open Nos. 59-49544 and 59-166959, deposit films of titanyloxyphthalocyanine to be used as charge generation layers are disclosed. However, the deposition for preparing a charge generation layer is not appropriate for the mass production because it requires the large investment in plant and equipment such as vacuum deposition system and results in costly production of electrophotographic photoreceptor.
For the industrial production, therefore, it is preferable to make a charge generation layer by a coating method comprising the steps of dispersing and dissolving both a charge generation material and a binder in a solvent to prepare a coating solution and applying the obtained solution on a surface of a substrate to form the charge generation layer. However, the photoreceptor having the charge-generation layer provided as a film formed by the coating method is not advantageous to the one obtained by the deposition method with respect to their electric characteristics, image-characteristics, and the changes in the characteristics by repeat use.
Comparing with the photoreceptor obtained by the deposition method, the photoreceptor obtained by the coating method shows a low charge retentivity, a high residual potential, and almost 40 percent low spectral sensitivity (e.g., one of the examples is described in an official gazette of Japanese Patent Application Laying-open No. 59-49544). About the cause, the charge-generation material does not work uniformly and effectively in the coated charge-generation layer being formed unevenly by an effect of settling out or aggregating the charge-generation material as a result of the unstable dispersion of the charge-generation material being dispersed in the solvent. Besides, the trapping of charge carriers and the dropping of photo sensitivity are observed at the portion where the content of the charge generation material is lowered. During the repeat use of the photoreceptor, the photo sensitivities are changed and image ghost and memory phenomena are generated. In addition, another portion where the charge generation material is comparatively concentrated may be responsible for the image noise because of the difference in electric properties between the portion where the charge generation material is comparatively concentrated and its surroundings.
Accordingly, these phthalocyanines must be contented with many required characteristics not only their electrical characteristics but also their dispersion stabilities and the like, for using them as charge-generation materials in the coating method for preparing a photoreceptor.
Regarding the electrical characteristics in an initial state, it must be of having excellent electrical characteristics such as an excellent photosensitivity, an excellent static electrification, and a small residual potential, and also it must be stable in darkness without causing gradual deterioration to an inferior state. It is further required that the photoreceptor must be of having a cycle stability, i.e., keeping its electrical characteristics in stable during and after the period of using the photoreceptor repeatedly. Furthermore, the charge-generation material must be of keeping a dispersion stability without causing any troubles such as coagulation, sedimentation, and crystal growth in a coating solution for a long time.
It has been known that the electrical characteristics of phthalocyanine depends largely on its coordinated metal species. Also, the phthalocyanine shows different properties of static electrification, dark attenuation, photosensitivity, and the like in accordance with not only the type of coordinated metal but also its crystal form (Sawada, M., Japanese Journal of "DYES and CHEMICALS", vol. 24, No. 6, pages 122-128, 1979).
Regarding a dispersion stability of phthalocyanine pigment, on the other hand, it has been known that it depends on a particle size, a particle form, and a surface .xi.-potential (Kumano, I. Denshi Shashin Gakkai-shi, pages 111-120, No. 2, vol. 22, 1984). In this case, the particle form and the quantum surface potential are also affected by various kinds of the crystal form (i.e., habit) depending on the extent of a growth of crystallographically equivalent surfaces. Therefore, it is very important to develop a crystal particle having a habit suitable for providing an excellent electrical characteristics and an excellent dispersion stability. Conventionally, it has been possible to develop phthalocyanine crystals having crystal forms for providing good electrical characteristics but habits suitable for providing dispersion stabilities.
In the case of the same materials having different habits, by the way, it is possible to distinguish between the two by means of external forms of their crystals. However, they can be distinguished more precisely by making a comparison between their patterns of X-ray diffraction spectra to be obtained by performing a X-ray powder method or the like.
That is, the process includes the steps of obtaining a X-ray diffraction spectrum of each crystal sample and making a comparison between their diffraction intensities with respect to each angle of diffraction.
As listed in Table 1 below, the titanylphthalocyanine crystals are classified into several structure types in accordance with their X-ray diffraction spectra, and also their lattice constants are determined by means of the structure analysis.
TABLE 1 ______________________________________ Crystal type Lattice constant Reference ______________________________________ .alpha. Phase II JP61-217050 JP61-239348 JP62-134651 I Phase I JP62-67094 II JP62-67094 A Phase I JP63-364, JP63-366 B Phase II JP63-364, JP63-366 C JP63-364, JP63-366 m C JP63-198067 amorphous JP1-123868 ______________________________________
FIGS. 1 to 4 are X-ray diffraction spectra of titanyloxyphthalocyanine crystals in the types of I, IT, .alpha., and amorphous, respectively. That is, FIG. 1 is an X-ray diffraction spectrum of titanyloxyphthalocyanine crystal (type I) described in Japanese Patent Application Laying-open No. 62-67094; FIG. 2 is an X-ray diffraction spectrum of titanyloxyphthalocyanine crystal (type II) described in Japanese Patent Application Laying-open No.62-67094; FIG. 3 is an X-ray diffraction spectrum of titanyloxyphthalocyanine crystal (type .alpha.) described in Japanese Patent Application Laying-open No.62-134651; and FIG. 4 is an X-ray diffraction spectrum of amorphous titanyloxyphthalocyanine crystal.
Regarding the above lattice constants listed in Table 1, phase II type crystals are grouped in triclinic crystals, while phase I and C type crystals are grouped in monoclinic crystals.
The above clarification is also explained in Journal of Imaging Science and Technology, 605-609, vol. 37, No. 6, 1993.
Furthermore, there is another way for classifying titanyloxyphthalocyanine crystals in accordance with Bragg angles and intensities in their X-ray diffraction spectra. That is, they are classified into the following four groups.
However, the titanyloxyphthalocyanines of the above groups have some disadvantages in practical use. That is, for example, the photoreceptor comprising one of them does not show a sufficient photo sensitivity against a beam of laser, a sufficient charging ability, a sufficient potential stability under the condition of recycling use; and a potential stability under the changes in use conditions.
In addition, there is another disadvantage of an insufficient dispersion stability when the above typed titanyloxyphthalocyanine crystal is used as a charge generation material to be dispersed in a solvent to prepare a coating solution. For that reason, in the case of forming charging generation layers by using the coating solution immediately after the dispersion or after passing times, the changes in electrophotographic characteristics of the resultant photoreceptors can be observed in spite of using the same coating condition. In this case, furthermore, there are other troubles such as an image noise to be caused by aggregated particles. As a result, it is very difficult to obtain an electrophotographic photoreceptor which is of good quality and of excellent characteristics in industrial and economical terms.
In general, by the way, a charge transport material preferable for a certain charge generation material is not always good for other charge generation materials. In addition, a charge generation material preferable for a certain charge transport material is not always good for other charge transport materials. That is, it means that there is an appropriate combination between them. Thus an inappropriate combination between them leads to several problems in practical use, such as insufficient sensitivities, low static electrification, and unstable electrical characteristics in repeat use of the photoreceptor.
As described above, the above combination is a matter of great importance to the photoreceptor. However, there is no definite theory concerned about the combination. Therefore the appropriate combination of the above materials has been investigated by way of experiment. It means that the appropriate combination of the charge generation material and the charge transport material have been hardly found.
In the case of the laminate-type photoreceptor, furthermore, there is an under-coating layer mainly consisting of a resin between the conductive layer and the photosensitive layer for preventing the injection of charge carriers from the conductive substrate. The reasons of applying the under-coating layer on the substrate is for forming a good photosensitive layer without causing unevenness thereof. That is, the under-coating layer covers defects of substrate's surface, such as an irregular shape, impurities and roughness thereof.
The resin to be used for the under-coating layer can be selected from the group of solvent-soluble polyamide, polyvinyl alcohol, polyvinyl butyral, casein, and so on. It is possible to prevent the injection of charge carriers by processing the resin into a thin film of under 0.5 .mu.m in thickness. In general, however, the under-coating layer should be formed as a film of over 0.5 .mu.m in thickness for covering a surface of the conductive substrate and forming a photoconductive layer without unevenness. As the case may be, the under-coating layer should be formed as a film of over 1 .mu.m in thickness, for example in accordance with the conditions of processing the substrate and the conditions of impurities on the substrate's surface. However, if the resin layer is prepared as a thick film made of the above resin such as polyvinyl alcohol, polyvinyl butyral, and casein, it shows large electrical resistivity and high residual potential of the photoreceptor. In this case, furthermore, there is another problem of the big change in electrical properties of the photoreceptor under the condition of low temperature and low humidity or the condition of high temperature and high humidity because of the following reasons. That is, water-absorption properties of the resin layer is comparatively large and thus the water content thereof is largely changed in accordance with the surrounding conditions. In addition, the electrical conductivity of the resin layer is depend on the movement of H.sup.+ ions and OH.sup.- ions to be generated by dissociating water molecules in the resin (i.e., ionic conductivity).
Currently, various materials have been proposed as an appropriate material to be used as an under-coating layer having low electric resistivity whether the layer is thick or not. For example, Japanese Patent Laying-open No. 2-193152, Japanese Patent Laying-open No. 3-288157, and Japanese Patent Laying-open No. 4-31870 disclose the specific resin structures of solvent-soluble polyamide resins. Japanese Patent Laying-open No. 3-145652, Japanese Patent Laying-open No. 3-81788, and Japanese Patent Laying-open No. 2-281262 disclose the mixtures of polyamide resin with other resins in expectation of reducing the effects of environmental changes by adjusting the electric resistance. However, these materials cannot avoid the effects of thermal and humidal conditions because of their compositions mainly including polyamide resin.